The present invention relates generally to internal combustion engines operable for discharging exhaust gas to electrically heated catalysts and, more particularly, to a method and system for distributing electrical energy from an integrated starter-alternator during a deceleration or overrun vehicle condition to an electrically heated catalyst in order to maintain the temperature of the electrically heated catalyst within an operating temperature range.
For a typical vehicle powered by a sparked ignited internal combustion engine, two of the most difficult operating conditions for controlling exhaust emissions are deceleration and overrun. During vehicle deceleration, the engine is producing very little power and is being partly motored by the vehicle. xe2x80x9cOverrunxe2x80x9d is an expression that also describes a deceleration condition, but additionally may describe a condition where a vehicle is coasting down a sustained downgrade with the engine being partly motored by the vehicle.
Herein, xe2x80x9coverrunxe2x80x9d will be used to include the deceleration condition. In either case, the engine load is very light and the percentage of internal exhaust gas re-circulation (EGR) is high. Both of these factors contribute to marginal combustion stability. This results in high engine out hydrocarbon (HC) emissions and cool exhaust gas temperatures. The low exhaust gas temperatures are particularly a problem for the catalyst which are often not hot enough to keep the catalyst active. The engine calibrator is faced with either of two options each of which has its own drawbacks.
The first option is to leave the fuel injection enabled during the overrun. In this case the cool exhaust temperature will cool the catalyst causing it to become inefficient or even inactive. This allows hydrocarbons to accumulate in the catalyst or exit the tailpipe untreated. This problem is exacerbated by combustion stability being marginal under this condition. Hence, the engine out HC emissions tend to be exceptionally high. Once the engine comes back under load after a sustained overrun condition, the catalyst must be heated by the hot exhaust gas for the catalyst to become active and efficient again. Until that happens, untreated hydrocarbons will leave the tailpipe. In addition, the unburned hydrocarbons which accumulated in the catalyst during the overrun can create a run away thermal reaction once the engine comes under load again. This can either shorten the life of the catalyst or destroy the catalyst.
Another compromise with this option is that spark advance is typically retarded during an overrun condition. This is an attempt to keep the catalyst hot by initiating combustion late thereby increasing exhaust gas temperature. However, this compromises combustion efficiency and increases fuel consumption.
The second option is to disable the fuel injection during the overrun condition. This prevents unburned hydrocarbons from accumulating in the catalyst. However, the engine is now being motored completely by the vehicle and pumps cool ambient air through the catalyst which cools the catalyst. When the vehicle comes off of the overrun condition and the fuel injection is enabled again, the catalyst temperature may be close to ambient and will have to be reheated a great amount before becoming active again. During this time interval when the catalyst is warming up large amounts of untreated hydrocarbons may escape out of the tailpipe.
Accordingly, what is needed is a method and system operable with an electrically heated catalyst for addressing problems which occur in achieving low exhaust emissions and low fuel consumption when a vehicle goes into a sustained deceleration or overrun operating condition.
An object of the present invention is to provide a method and system for distributing electrical energy from an integrated starter-alternator during a deceleration or overrun vehicle condition to an electrically heated catalyst in order to maintain the temperature of the electrically heated catalyst within an operating temperature range.
In accordance with the object and other objects, the present invention provides an exhaust system for a motor vehicle having an internal combustion engine. The exhaust system includes an electrically heated catalyst operable for cleaning exhaust gas discharged by the internal combustion engine when the temperature of the electrically heated catalyst is in an operating temperature range. The exhaust system further includes a catalyst heater which converts electrical energy into heat. An integral starter-alternator converts mechanical energy generated by the internal combustion engine into electrical energy. The electrical energy from the integral starter-alternator is supplied to the catalyst heater during vehicle overrun. The catalyst heater uses the electrical energy to heat the electrically heated catalyst in order to maintain the temperature of the electrically heated catalyst in the operating temperature range during vehicle overrun.
Further, in accordance with the object and other objects, the present invention also provides a method for cleaning exhaust gas generated by an internal combustion engine of a motor vehicle during vehicle overrun. The method includes cleaning exhaust gas discharged by the internal combustion engine with an electrically heated catalyst when the temperature of the electrically heated catalyst is in an operating temperature range. An integral starter-alternator is then used to convert mechanical energy generated by the internal combustion engine and kinetic energy of motion of the vehicle into electrical energy. The electrical energy from the integral starter-alternator is then converted into heat by the electrically heated catalyst in order to maintain the temperature of the electrically heated catalyst in the operating temperature range during vehicle overrun.
There are numerous advantages associated with the present invention. As well as contributing a minor amount of braking action, the present invention helps the catalyst maintain its temperature and remain active and efficient. This eliminates the need to consider the second option described above where fuel is disabled during the overrun with its associated emission penalties. The present invention also eliminates the drawbacks associated with the first option because electrical heating of the catalyst during an overrun condition allows the catalyst to remain active and efficient.